Using optical metrology for within wafer feed forward process control

ABSTRACT

A method of controlling the polishing of a substrate includes polishing a substrate on a first platen using a first set of parameters, obtaining first and second sequences of measured spectra from first and second regions of the substrate with an in-situ optical monitoring system, generating first and second sequences of values from the first and second sequences of measured spectra, fitting first and second linear functions to the first and second sequences of values, determining a difference between the first linear function and the second linear function, adjusting at least one parameter of a second set of parameters based on the difference, and polishing the substrate on a second platen using the adjusted parameter.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Provisional Application Ser.No. 61/118,135, filed on Nov. 26, 2008.

BACKGROUND

1. Technical Field

This specification relates to the monitoring and control of a chemicalmechanical polishing process.

2. Description of the Related Art

Sub-quarter micron multi-level metallization is one of the keytechnologies for the next generation of ultra large-scale integration(ULSI). The multilevel interconnects that lie at the heart of thistechnology require planarization of interconnect features formed in highaspect ratio apertures, including contacts, vias, trenches and otherfeatures. Reliable formation of these interconnect features is veryimportant to the success of ULSI and to the continued effort to increasecircuit density and quality on individual substrates and die.

In the fabrication of integrated circuits and other electronic devices,multiple layers of conducting, semiconducting, and dielectric materialsare deposited on or removed from a surface of a substrate. Thin layersof conducting, semiconducting, and dielectric materials may be depositedby a number of deposition techniques. Common deposition techniques inmodern processing include physical vapor deposition (PVD), also known assputtering, chemical vapor deposition (CVD), plasma-enhanced chemicalvapor deposition (PECVD), and electrochemical plating (ECP).

As layers of materials are sequentially deposited and removed, theuppermost surface of the substrate may become non-planar across itssurface and require planarization. An example of a non-planar process isthe deposition of copper films with an ECP process in which the coppertopography simply follows the already existing non-planar topography ofthe wafer surface, especially for lines wider than 10 microns.Planarizing a surface, or “polishing” a surface, is a process wherematerial is removed from the surface of the substrate to form agenerally even, planar surface. Planarization is useful in removingundesired surface topography and surface defects, such as roughsurfaces, agglomerated materials, crystal lattice damage, scratches, andcontaminated layers or materials. Planarization is also useful informing features on a substrate by removing excess deposited materialused to fill the features and to provide an even surface for subsequentlevels of metallization and processing.

Chemical Mechanical Planarization, or Chemical Mechanical Polishing(CMP), is a common technique used to planarize substrates. CMP utilizesa chemical composition, such as slurries or other fluid medium, forselective removal of materials from substrates. In conventional CMPtechniques, a substrate carrier or polishing head is mounted on acarrier assembly and positioned in contact with a polishing pad in a CMPapparatus. The carrier assembly provides a controllable pressure to thesubstrate, thereby pressing the substrate against the polishing pad. Thepad is moved relative to the substrate by an external driving force. TheCMP apparatus affects polishing or rubbing movements between the surfaceof the substrate and the polishing pad while dispersing a polishingcomposition to affect chemical activities and/or mechanical activitiesand consequential removal of materials from the surface of thesubstrate.

An objective of CMP is to remove a predictable amount of material whileachieving uniform surface topography both within each wafer and fromwafer to wafer when performing a batch polishing process.

Therefore, there is a need for a polishing process which accurately andreliably removes a predictable amount of material while achievinguniform surface topography.

SUMMARY

In one aspect, a method of controlling the chemical mechanical polishingof a substrate includes polishing a substrate on a first platen using afirst set of polishing parameters, obtaining a plurality of measuredspectra from at least two zones of the substrate by in-situ opticalmonitoring during polishing, comparing the plurality of measured spectrawith a reference spectrum to evaluate the thickness of each of the atleast two zones of the substrate, comparing a thickness of a first zoneof the at least two zones with a thickness of a second zone of the atleast two zones, determining whether the thickness of the first zone ofthe at least two zones falls within a predetermined range of thethickness of the second zone of the at least two zones, and if thethickness does not fall within the predetermined range, at least one ofa) adjusting at least one polishing parameter of the first set ofpolishing parameters and polishing a second substrate on the firstplaten using the adjusted polishing parameters, or b) adjusting at leastone polishing parameter of a second set of polishing parameters andpolishing the substrate on a second platen using the adjusted polishingparameters.

Implementations can include one or more of the following features.

A plurality of measured spectra may be obtained from at least two zonesof the second substrate by in-situ optical monitoring during polishing,the plurality of measured spectra may be compared with a referencespectrum to evaluate the thickness of each of the at least two zones ofthe second substrate, and a thickness of a first zone of the at leasttwo zones of the second substrate may be compared with a thickness of asecond zone of the at least two zones.

Whether the thickness of the first zone of the at least two zones fallswithin a predetermined range of the thickness of the second zone of theat least two zones may be determined, and if the thickness does not fallwithin the predetermined range, at least one of re-adjusting at leastone polishing parameter of the first set of polishing parameters andpolishing a third substrate on the first platen using the re-adjustedpolishing parameters, or b) adjusting at least one polishing parameterof the second set of polishing parameters and polishing the secondsubstrate on a second platen using the adjusted polishing parameters.

Obtaining a plurality of measured spectra from at least two zones of thesubstrate may include illuminating the substrate with white light andobtaining spectra of light reflected from the substrate.

Adjusting the polishing parameter may include adjusting the polishingpressure.

The thickness may be a relative thickness, an index value, and/or afinal thickness.

Evaluating the thickness of the substrate may be performed using robustline fitting techniques.

At least one polishing parameter of the first set of polishingparameters may be adjusted and the second substrate may be polished onthe first platen using the adjusted polishing parameters.

At least one polishing parameter of the second set of polishingparameters may be adjusted, and the substrate may be polished on thesecond platen using the adjusted polishing parameters.

In another aspect, a method of controlling the chemical mechanicalpolishing of a substrate includes polishing a substrate on a firstplaten using a first set of polishing parameters, obtaining a firstsequence of measured spectra with an in-situ optical monitoring system,each measured spectrum from the first sequence of measured spectra beinga spectrum of light reflected from a first region of the substrate,generating a first sequence of values from the first sequence ofmeasured spectra, fitting a first linear function to the first sequenceof values, obtaining a second sequence of measured spectra with thein-situ optical monitoring system, each measured spectrum from thesecond sequence of measured spectra being a spectrum of light reflectedfrom a second region of the substrate, generating a second sequence ofvalues from the second sequence of measured spectra, fitting a secondlinear function to the second sequence of values, determining adifference between the first linear function and the second linearfunction, adjusting at least one polishing parameter of the first set ofpolishing parameters based on the difference so as to reduce an expecteddifference between an expected first linear function for the firstregion and an expected second linear function for the second regionduring polishing of a second substrate on the first platen, andpolishing the second substrate on the first platen using the adjustedpolishing parameter.

Implementations can include one or more of the following features.

The first region may be a control region and the second region may be areference region, and polishing of the first substrate at the firstplaten may be halted based on the second linear function reaching atarget value.

Determining the difference may include determining a first differencebetween a first time that the first linear function crosses a startingvalue and a second time that the second linear function reaches thetarget value. Determining the difference may further include determininga second difference between the target value and the starting value.

The polishing parameter may be pressure.

Adjusting the at least one polishing parameter may includes calculatinga desired polishing rate for the second substrate at the first platenfrom the first difference and the second difference.

MZD1 may equal (RE1−RSZ1)/(TE1−TZS1), where MZD1 is the desiredpolishing rate, RE1 is the target value, RSZ1 is the starting value, TE1is the second time that the second linear function reaches the targetvalue, and TZS1 is the first time that the first linear function crossesthe starting value.

Adjusting the polishing rate may include multiplying a pressure by aratio of the desired polishing rate for the second substrate at thefirst platen to the polishing rate of the substrate at the first platen.

The pressure may be the prior pressure for the control region forpolishing of the first substrate.

Adjusting the at least one polishing parameter may include adjusting apolishing parameter for the control region.

The difference in time may be measured as a difference in platenrotations.

Generating a first sequence of values may include, for each measuredspectrum, determining a best fitting reference spectrum from a libraryof reference spectra, and determining a value associated with the bestfitting reference spectrum.

The value comprises an index value or a thickness value.

Obtaining a plurality of measured spectra from at least two zones of thesubstrate may include illuminating the substrate with white light andobtaining spectra of light reflected from the substrate.

In another aspect, a method of controlling the chemical mechanicalpolishing of a substrate includes polishing a substrate on a firstplaten using a first set of polishing parameters, obtaining a firstsequence of measured spectra with an in-situ optical monitoring system,each measured spectrum from the first sequence of measured spectra beinga spectrum of light reflected from a first region of the substrate,generating a first sequence of values from the first sequence ofmeasured spectra, fitting a first linear function to the first sequenceof values, obtaining a second sequence of measured spectra with thein-situ optical monitoring system, each measured spectrum from thesecond sequence of measured spectra being a spectrum of light reflectedfrom a second region of the substrate, generating a second sequence ofvalues from the second sequence of measured spectra, fitting a secondlinear function to the second sequence of values, determining adifference between the first linear function and the second linearfunction, adjusting at least one polishing parameter of a second set ofpolishing parameters based on the difference so as to reduce an expecteddifference between an expected first linear function for the firstregion and an expected second linear function for the second regionduring polishing of the substrate on the second platen, and polishingthe substrate on the second platen using the adjusted polishingparameter.

Implementations can include one or more of the following features.

The first region may be a control region and the second region may be areference region, and polishing of the first substrate at the firstplaten may be halted based on the second linear function reaching atarget value.

Determining the difference may include determining a difference betweena first time that the first linear function reaches the target value anda second time that the second function reaches the target value.

Adjusting the at least one polishing parameter may include multiplyingthe difference in time by a ratio of the rotation rate of the firstplaten to the rotation rate of the second platen.

The polishing parameter may be pressure.

Adjusting the at least one polishing parameter may include calculatingan expected polishing rate for the substrate at the second platen from asecond target value, a second target time and the difference in time.

MZD2 may equal RE2/(TE2−TZS2), where MZD2 is the expected polishingrate, RE2 is the second target value, TE2 is the second target time, andTZS2 is the difference in time multiplied by the ratio of the rotationrate of the first platen to the rotation rate of the second platen.

Adjusting the polishing rate may include multiplying a pressure by aratio of expected polishing rate to a prior polishing rate.

The pressure may be a pressure for the reference region.

Adjusting the at least one polishing parameter may include adjusting apolishing parameter for the control region.

The difference in time may be measured as a difference in platenrotations.

Generating a first sequence of values may include, for each measuredspectrum, determining a best fitting reference spectrum from a libraryof reference spectra, and determining a value associated with the bestfitting reference spectrum.

The value may be an index value or a thickness value.

Obtaining a plurality of measured spectra from at least two zones of thesubstrate may include illuminating the substrate with white light andobtaining spectra of light reflected from the substrate.

In another aspect, a computer program product, tangibly embodied on acomputer readable media, may include instructions for causing aprocessor to control a chemical mechanical polisher to performoperations of any of the methods forth above.

In another aspect, a system for chemical mechanical polishing mayinclude a first rotatable platen for supporting a polishing surface, alight source in the platen, a light detector in the platen, a carrierhead configured to hold a substrate against the polishing surface andmove the substrate so that light from the light source is directed ontothe surface of the substrate and light reflected from the substrate isdetected by the light detector, a controller configured to receive asignal from the light detector, wherein the controller is furtherconfigured to perform operations of any of the methods set forth above.

BRIEF DESCRIPTION OF THE DRAWINGS

So that the manner in which the above recited features can be understoodin detail, a more particular description, briefly summarized above, maybe had by reference to various implementations, some of which areillustrated in the appended drawings. It is to be noted, however, thatthe appended drawings illustrate only typical implementations and aretherefore not to be considered limiting of the scope of the claims, forthere may be other equally effective implementations.

FIG. 1 is a schematic cross-sectional view of a chemical mechanicalpolishing apparatus;

FIG. 2 is a schematic cross-sectional view of a polishing station;

FIG. 3 is an overhead view of a substrate on a platen and showslocations where measurements are taken;

FIG. 4 illustrates a method for polishing a substrate;

FIG. 5 illustrates another method for polishing a substrate;

FIG. 6A illustrates a graph of polishing progress versus time for aprocess in which the polishing rates are adjusted;

FIG. 6B illustrates a graph of polishing progress versus time for aprocess in which the polishing rates are adjusted;

FIG. 7A illustrates a graph of the polishing progress versus time for aprocess in which the polishing rates are adjusted; and

FIG. 7B illustrates a graph of the polishing progress versus time for aprocess in which the polishing rates are adjusted.

To facilitate understanding, identical reference numerals have beenused, where possible, to designate identical elements that are common tothe figures. It is contemplated that elements disclosed in oneimplementation may be beneficially utilized on other implementationswithout specific recitation.

DETAILED DESCRIPTION

Implementations described herein relate to methods and optical systemsfor polishing substrates and determining relative thickness and apolishing endpoint. An optical detector is used to obtain spectra from asubstrate during polishing on a platen. The spectra are compared tospectra in a library. The comparison can be done using varioustechniques, such as the least sum of squared matching techniques, whichare described in U.S. Patent Publication No. 2007-0042675, and U.S.Patent Publication No. 2007-0224915. If each spectrum in the library isassigned an index number, the matching index numbers can be plottedaccording to time and a line fit to the plotted index numbers usingrobust line fitting techniques, which are described in U.S. PatentPublication No. 2008-0206993. When the line intersects the indexcorresponding to a target spectrum the target endpoint is reached andpolishing can be stopped.

Information regarding the relative thickness and endpoint time ofvarious regions of the substrate can be used to adjust polishingparameters, such as the polishing pressure, for each defined region onthe substrate so that the different regions reach a target thickness atthe same time, to obtain a more uniform polish across the surface of thesubstrate when polishing is halted simultaneously across the substrate.However, such in-situ modification of polishing parameters may not workwell when polish times are short and/or there is not enough time due topoor sampling rates. The implementations described herein use theinformation regarding the relative thickness and endpoint time to modifypolishing parameters for both subsequent substrates polished on the sameplaten and for the same substrate as the substrate is polished onadditional platens.

In some implementations, the relative thickness and endpoint time basedon spectra of various regions of a substrate polished on a first platen(platen x) can be used to modify the polishing parameters for the samesubstrate as the substrate is polished on additional platens (platenx+1). In other implementations, the relative thickness and endpoint timebased on spectra of various regions of a first substrate polished on aplaten (platen x) can be used to modify the polishing parameters for asecond substrate polished on the same platen (platen x). In yet otherimplementations, the relative thickness and endpoint time based onspectra of various regions of a substrate polished on a first platen(platen x) is used in conjunction with the relative thickness andendpoint time based on the spectra of various regions of the samesubstrate polished on a second platen (x+1) and used to modify thepolishing parameters for subsequent substrates polished on the firstplaten and/or second platen. Gain factors and other signal processingcontrol techniques may be used to achieve better performance.

While the particular apparatus in which the implementations describedherein can be practiced is not limited, it is particularly beneficial topractice the implementations in a REFLEXION LK CMP system and MIRRAMESA® system sold by Applied Materials, Inc., Santa Clara, Calif.Additionally, CMP systems available from other manufacturers may alsobenefit from implementations described herein. Implementations describedherein may also be practiced on overhead circular track polishingsystems.

FIG. 1 shows a chemical mechanical polishing apparatus 20 that canpolish one or more substrates 10. Polishing apparatus 20 includes aseries of polishing stations 22 and a transfer station 23. Transferstation 23 transfers the substrates between carrier heads 70 and aloading apparatus (not shown).

Each polishing station 22 includes a rotatable platen 24 on which isplaced a polishing pad 30. The first and second stations 22 can includea two-layer polishing pad with a hard durable outer surface or afixed-abrasive pad with embedded abrasive particles. The final polishingstation 22 can include a relatively soft pad. Each polishing station 22can also include a pad conditioner apparatus 28 to maintain thecondition of the polishing pad so that it will effectively polishsubstrates 10.

A rotatable multi-head carousel 60 supports four carrier heads 70. Thecarousel 60 is rotated by a central post 62 about a carousel axis 64 bya carousel motor assembly (not shown) to orbit the carrier heads 70 andthe substrates 10 attached thereto between polishing stations 22 andtransfer station 23. Three of the carrier heads 70 receive and holdsubstrates 10, and polish them by pressing them against the polishingpads 30. Meanwhile, one of the carrier heads 70 receives a substrate 10from and delivers a substrate 10 to transfer station 23.

Each carrier head 70 is connected by a carrier drive shaft 74 to acarrier head rotation motor 76 (shown by the removal of one quarter ofcover 68) so that each carrier head 70 can independently rotate aboutits own axis. In addition, each carrier head 70 independently laterallyoscillates in a radial slot 72 formed in carousel support plate 66. Adescription of a suitable carrier head 70 can be found in U.S. Pat. No.6,422,927.

A slurry 38 containing a reactive agent (e.g., deionized water for oxidepolishing) and a chemically-reactive catalyzer (e.g., potassiumhydroxide for oxide polishing) can be supplied to the surface of thepolishing pad 30 by a slurry supply port or a combined slurry/rinse arm39. If the polishing pad 30 is a standard pad, the slurry 38 can alsoinclude abrasive particles (e.g., silicon dioxide for oxide polishing).An optical access, for example, a window 36, is included in thepolishing pad 30 and is positioned such that it passes beneath substrate10 during a portion of the platen's rotation, regardless of thetranslational position of the carrier head 70. In certain the window 36and related sensing methods may be used for an endpoint detectionprocess.

To facilitate control of the polishing apparatus 20 and processesperformed thereon, a controller 90 comprising a central processing unit(CPU) 92, memory 94, and support circuits 96, is connected to thepolishing apparatus 20. The CPU 92 may be one of any form of computerprocessor that can be used in an industrial setting for controllingvarious drives and pressures. The memory 94 is connected to the CPU 92.The memory 94, or computer-readable medium, may be one or more ofreadily available memory such as random access memory (RAM), read onlymemory (ROM), floppy disk, hard disk, or any other form of digitalstorage, local or remote. The support circuits 96 are connected to theCPU 292 for supporting the processor in a conventional manner. Thesecircuits include cache, power supplies, clock circuits, input/outputcircuitry, subsystems, and the like.

FIG. 2 is a schematic cross-sectional view of a chemical mechanicalpolishing station 22 operable to polish a substrate 10. The polishingstation 22 includes a rotatable disk-shaped platen 24, on which apolishing pad 30 is situated. The platen 24 is operable to rotate aboutan axis 25. For example, a motor (not shown) can turn a drive shaft 27to rotate the platen 24. The polishing pad 30 can be detachably securedto the platen 24, for example, by a layer of adhesive. When worn, thepolishing pad 30 can be detached and replaced. The polishing pad 30 canbe a two-layer polishing pad with an outer polishing layer 32 and asofter backing layer 34.

Optical access through the polishing pad 30 is provided by including anaperture (i.e., a hole that runs through the pad) or a solid window. Thesolid window can be secured to the polishing pad 30, although in someimplementations the solid window can be supported on the platen 24 andproject into an aperture in the polishing pad 30. The polishing pad 30is usually placed on the platen 24 so that the aperture or windowoverlies an optical head 53 situated in a recess 26 of the platen 24.The optical head 53 consequently has optical access through the apertureor window to a substrate being polished. The optical 53 head is furtherdescribed below.

The window 36 can be, for example, a rigid crystalline or glassymaterial, e.g., quartz or glass, or a softer plastic material, e.g.,silicone, polyurethane or a halogenated polymer (e.g., a fluoropolymer),or a combination of the materials mentioned. The window 36 can betransparent to white light. If a top surface of the solid window 36 is arigid crystalline or glassy material, then the top surface should besufficiently recessed from the polishing surface to prevent scratching.If the top surface is near and may come into contact with the polishingsurface, then the top surface of the window 36 should be a softerplastic material. In some implementations the solid window 36 is securedin the polishing pad 30 and is a polyurethane window, or a window 36having a combination of quartz and polyurethane. The window 36 can havehigh transmittance, for example, approximately 80% transmittance, formonochromatic light of a particular color, for example, blue light orred light. The window 36 can be sealed to the polishing pad 30 so thatliquid does not leak through an interface of the window 36 and thepolishing pad 30.

A bottom surface of the window 36 can optionally include one or morerecesses. A recess can be shaped to accommodate, for example, an end ofan optical fiber cable or an end of an eddy current sensor. The recessallows the end of the optical fiber cable or the end of the eddy currentsensor to be situated at a distance, from a substrate surface beingpolished, that is less than a thickness of the window 36. With animplementation in which the window 36 includes a rigid crystallineportion or glass like portion and the recess is formed in such a portionby machining, the recess is polished so as to remove scratches caused bythe machining. Alternatively, a solvent and/or a liquid polymer can beapplied to the surfaces of the recess to remove scratches caused bymachining. The removal of scratches usually caused by machining reducesscattering and can improve the transmittance of light through the window36. Implementations of the window 36 are described in U.S. PatentPublication No. 2007/0042675.

The above described window 36 and polishing pad 30 can be manufacturedusing a variety of techniques. The backing layer 34 of the polishing pad30 can be attached to its outer polishing layer 32, for example, byadhesive. The aperture that provides optical access can be formed in thepad 30, e.g., by cutting or by molding the pad 30 to include theaperture, and the window 36 can be inserted into the aperture andsecured to the pad 30, e.g., by an adhesive. Alternatively, a liquidprecursor of the window 36 can be dispensed into the aperture in the pad30 and cured to form the window 36. Alternatively, a solid transparentelement, e.g., the above described crystalline or glass like portion,can be positioned in liquid pad material, and the liquid pad materialcan be cured to form the pad 30 around the transparent element. Ineither of the later two cases, a block of pad material can be formed,and a layer of polishing pad 30 with the molded window 36 can be scythedfrom the block.

The polishing station 22 can include a flushing system to improve lighttransmission through the optical access. There are differentimplementations of the flushing system. With implementations of thepolishing station 22 in which the polishing pad 30 includes an apertureinstead of a solid window, the flushing system is implemented to providea laminar flow of a fluid, e.g., a gas or liquid, across a top surfaceof the optical head 53. (The top surface can be a top surface of a lensincluded in the optical head 53.) The laminar flow of fluid across thetop surface of the optical head 53 can sweep opaque slurry out of theoptical access and/or prevent slurry from drying on the top surface and,consequently, improves transmission through the optical access. Withimplementations in which the polishing pad 30 includes a window 36instead of an aperture, the flushing system is implemented to direct aflow of gas at a bottom surface of the window 36. The flow of gas canprevent condensation from forming at the bottom surface of the window 36which would otherwise impede optical access.

Returning to FIG. 1, the polishing station 22 includes a combinedslurry/rinse arm 39. During polishing, the arm 39 is operable todispense slurry 38 containing a liquid and a pH adjuster. Alternatively,the polishing station 22 includes a slurry port operable to dispenseslurry onto polishing pad 30.

The polishing station 22 includes the carrier head 70 operable to holdthe substrate 10 against the polishing pad 30. The carrier head 70 issuspended from a support structure 73, for example, a carousel 60, andis connected by a carrier drive shaft 74 to a carrier head rotationmotor 76 so that the carrier head can rotate about an axis 71. Inaddition, the carrier head 70 can oscillate laterally in a radial slotformed in the support structure 73. In operation, the platen 24 isrotated about its central axis 25, and the carrier head 70 is rotatedabout its central axis 71 and translated laterally across the topsurface of the polishing pad 30.

The polishing station 22 also includes an optical monitoring system,which can be used to determine a polishing endpoint as discussed below.The optical monitoring system includes a light source 51 and a lightdetector 52. Light passes from the light source 51, through the window36 in the polishing pad 30, impinges and is reflected from the substrate10 back through the window 36, and travels to the light detector 52.

A bifurcated optical cable 54 can be used to transmit the light from thelight source 51 to the window 36 and back from the window 36 to thelight detector 52. The bifurcated optical cable 54 can include a “trunk”55 and two “branches” 56 and 58.

As mentioned above, the platen 24 includes the recess 26, in which theoptical head 53 is situated. The optical head 53 holds one end of thetrunk 55 of the bifurcated fiber cable 54, which is configured to conveylight to and from a substrate surface being polished. The optical head53 can include one or more lenses or a window overlying the end of thebifurcated fiber cable 54. Alternatively, the optical head 53 can merelyhold the end of the trunk 55 adjacent the window in the polishing pad.The optical head 53 can hold the above-described nozzles of the flushingsystem. The optical head 53 can be removed from the recess 26 asrequired, for example, to effect preventive or corrective maintenance.

The platen 24 includes a removable in-situ monitoring module 50. Thein-situ monitoring module 50 can include one or more of the following:the light source 51, the light detector 52, and circuitry for sendingand receiving signals to and from the light source 51 and light detector52. For example, the output of the detector 52 can be a digitalelectronic signal that passes through a rotary coupler, e.g., a slipring, in the drive shaft 74 to the controller 90 for the opticalmonitoring system. Similarly, the light source can be turned on or offin response to control commands in digital electronic signals that passfrom the controller 90 through the rotary coupler to the module 50.

The in-situ monitoring module can also hold the respective ends of thebranch portions 56 and 58 of the bifurcated optical fiber cable 54. Thelight source is operable to transmit light, which is conveyed throughthe branch 56 and out the end of the trunk 55 located in the opticalhead 53, and which impinges on a substrate being polished. Lightreflected from the substrate is received at the end of the trunk 55located in the optical head 53 and conveyed through the branch 58 to thelight detector 52.

In some implementations, the bifurcated fiber cable 54 is a bundle ofoptical fibers. The bundle includes a first group of optical fibers anda second group of optical fibers. An optical fiber in the first group isconnected to convey light from the light source 51 to a substratesurface being polished. An optical fiber in the second group isconnected to receive light reflecting from the substrate surface beingpolished and convey the received light to a light detector. The opticalfibers can be arranged so that the optical fibers in the second groupform an X-like shape that is centered on the longitudinal axis of thebifurcated optical fiber (as viewed in a cross section of the bifurcatedfiber cable 54). Alternatively, other arrangements can be implemented.For example, the optical fibers in the second group can form V-likeshapes that are mirror images of each other. A suitable bifurcatedoptical fiber is available from Verity Instruments, Inc. of Carrollton,Tex.

There is usually an optimal distance between the window 36 of thepolishing pad 30 and the end of the trunk 55 of bifurcated fiber cable54 proximate to the window 36 of the polishing pad 30. The distance canbe empirically determined and is affected by, for example, thereflectivity of the window 36, the shape of the light beam emitted fromthe bifurcated fiber cable, and the distance to the substrate beingmonitored. In some implementations, the bifurcated fiber cable issituated so that the end proximate to the window 36 is as close aspossible to the bottom of the window 36 without actually touching thewindow 36. With this implementation, the polishing station 22 caninclude a mechanism, e.g., as part of the optical head 53, that isoperable to adjust the distance between the end of the bifurcated fibercable 54 and the bottom surface of the polishing pad window 36.Alternatively, the proximate end of the bifurcated fiber cable isembedded in the window 36.

The light source 51 is operable to emit white light. In someimplementations, the white light emitted includes light havingwavelengths of 200-800 nanometers. A suitable light source is a xenonlamp or a xenon-mercury lamp.

The light detector 52 can be a spectrometer. A spectrometer is basicallyan optical instrument for measuring properties of light, for example,intensity, over a portion of the electromagnetic spectrum. A suitablespectrometer is a grating spectrometer. Typical output for aspectrometer is the intensity of the light as a function of wavelength.

Optionally, the in-situ monitoring module 50 can include other sensorelements. The in-situ monitoring module 50 can include, for example,eddy current sensors, lasers, light emitting diodes, and photodetectors.With implementations in which the in-situ monitoring module 50 includeseddy current sensors, the module 50 is usually situated so that asubstrate being polished is within working range of the eddy currentsensors.

The light source 51 and light detector 52 are connected to thecontroller 90 to control their operation and to receive their signals.With respect to control, the controller 90 can, for example, synchronizeactivation of the light source 51 with the rotation of the platen 24. Asshown in FIG. 3, the controller 90 can cause the light source 51 to emita series of flashes starting just before and ending just after thesubstrate 10 passes over the in-situ monitoring module. (Each of points101-111 depicted represents a location where light from the in-situmonitoring module impinged and reflected off.) Alternatively, thecontroller 90 can cause the light source 51 to emit light continuouslystarting just before and ending just after the substrate 10 passes overthe in-situ monitoring module. Although not shown, each time thesubstrate 10 passes over the monitoring module, the alignment of thesubstrate 10 with the monitoring module can be different than in theprevious pass. Over one rotation of the substrate 10, spectra areobtained from different angular locations on the substrate 10, as wellas different radial locations. That is, some spectra are obtained fromlocations closer to the center of the substrate 10 and some are closerto the edge. The substrate 10 can be sectioned off into radial zones.Three, four, five, six, seven or more zones can be defined on thesurface of the substrate 10. In some implementations described herein,spectra are grouped into their corresponding zones.

With respect to receiving signals, the controller 90 can receive, forexample, a signal that carries information describing a spectrum of thelight received by the light detector 52. The controller 90 can processthe signal to determine an endpoint of a polishing step. Without beinglimited to any particular theory, the spectra of light reflected fromthe substrate 10 evolve as polishing progresses. Properties of thespectrum of the reflected light change as a thickness of the filmchanges, and particular spectrums are exhibited by particularthicknesses of the film. The controller 90 can execute logic thatdetermines, based on one or more of the spectra, when an endpoint hasbeen reached. The one or more spectra on which an endpoint determinationis based can include a target spectrum, a reference spectrum, or both.

As used in the instant specification, a target spectrum refers to aspectrum exhibited by the white light reflecting from a film of interestwhen the film of interest has a target thickness. By way of example, atarget thickness can be 1, 2, or 3 microns. Alternatively, the targetthickness can be zero, for example, when the film of interest is clearedso that an underlying film is exposed.

FIG. 4 illustrates a method 400 for polishing a substrate according toimplementations described herein. The method begins by polishing asubstrate 10 on a first platen 24 using a set of polishing parameters(step 402). The substrate 10 may have a material disposed thereon.Exemplary materials may include insulating materials, e.g., oxides, suchas silicon oxide. The polishing parameters may include, for example, oneor more of the platen rotational speed, the rotational speed of thecarrier head, the pressure or downward force applied by the carrier headto the substrate, material removal rate, carrier head sweep frequency,and slurry flow rate.

The substrate 10 is polished with the surface of the polishing pad 30.The substrate 10 is brought into contact with the polishing pad 30, moreparticularly, the material on the substrate is brought into contact withan upper surface of the polishing pad 30. The polishing pad 30 isrotated relative to the substrate 10, which is also rotated. In someimplementations, the polishing process may comprise a multi-steppolishing process. For example, bulk material may be removed on a firstplaten 24 using a high removal rate process with any residual conductivematerial removed on a second platen 24 using a “soft landing” or lowpressure/low removal rate process. In some implementations, thepolishing process may be performed on a single platen.

In some implementations, an incoming or pre-polish profile determinationis made, for example, by measuring the thickness of a particularsubstrate material across portions of the substrate 10. The profiledetermination may include determining the thickness profile of aconductive material across the surface of the substrate 10. A metricindicative of thickness may be provided by any device or devicesdesigned to measure film thickness of semiconductor substrates.Exemplary non-contact devices include iSCAN™ and iMAP™ available fromApplied Materials, Inc. of Santa Clara, Calif., which scan and map thesubstrate, respectively. The pre-polish profile determination may bestored in the controller 90.

A plurality of measured spectra is obtained from at least two zones ofthe substrate 10 by in-situ optical monitoring during the polishingprocess (step 404). The plurality of measured spectra can include aplurality, e.g., sequence, of measured spectra from each zone. In someimplementations, the substrate is divided into a series of zones. Insome implementations, the zones may comprise circular and annular zones,for example, the substrate may be divided into an annular edge zone, anannular middle zone, and a circular center zone. The plurality ofmeasured spectra may be obtained using the in-situ monitoring module 50as discussed herein. The plurality of measured spectra may be obtainedduring the polishing process in real-time.

The plurality of measured spectra is compared with a plurality ofreference spectra to evaluate the thickness of each of the at least twozones of the substrate 10 (step 406). The reference spectra may becollected while polishing a set-up substrate or series of set-upsubstrates using similar polishing parameters. The reference spectra maybe stored in a spectra library. Alternatively, the library can includespectra that are not collected but rather are generated based on theory.The library can be implemented in the memory of the controller 90 of thepolishing apparatus 20. Each reference spectrum in the library has anassociated value, such as an index value or a thickness value. Forexample, the reference spectra are indexed so that each referencespectrum has an index value, e.g., a unique index value, and the indexvalue associated with reference spectrum is stored, e.g., in computermemory. The index values for the reference spectra can be selected tomonotonically increase as polishing progresses, e.g., the index valuescan be proportional to a number of platen rotations or to time. Thuseach index value can be a whole number, and the index number canrepresent the expected platen rotation at which the associated referencespectrum would appear.

Alternatively, rather than storing index values, the computer memory canstore thickness values for the reference spectra, with each referencespectrum having an associated thickness value.

In certain implementations, for each measured spectrum, the referencespectrum stored in the library which best fits the measured spectrum isdetermined. The index value or thickness value associated with thedetermined reference spectrum is retrieved from memory. Thus, as asequence of measured spectra is obtained during polishing, a sequence ofindex values or thicknesses values is generated. For each index value orthickness value, the time or platen rotation at which the correspondingmeasured spectrum was measured can be stored. A linear function, e.g., afunction of time or platen rotation, can be fit to the index values orthickness values, e.g., using robust line fit techniques. The slope ofthe fitted line defines the polishing rate (in terms of index values orthickness per time or platen rotation). Where the fitted line crossesthe current time or platen rotation defines the present index or presentthickness for the layer on the substrate. Where the fitted line meetsthe target index or target thickness defines the endpoint time orrotation. Where the fitted line intersects the time or rotation at whichpolishing is halted, e.g., a target time or target rotation, defines afinal index value or final thickness value. In certain implementations,the index values or thickness values that match the current spectra areplotted, e.g., displayed on monitor, according to time or platenrotation.

The thickness of a first zone of the at least two zones is compared withthe thickness of a second zone of the at least two zones (step 408). Inthe context of methods 400 and 500, the thickness can be an actualthickness, e.g., a thickness value, or a stand-in for the actualthickness, e.g., an index value. In addition, the thickness can becalculated from a sequence, e.g., calculated from the line fit to thesequence of values, e.g., the thickness can be the present index orpresent thickness, or the thickness can simply be determined from justthe most recently measured spectrum, e.g., the thickness can be thethickness value or index value associated with the most recentlymeasured spectrum. In addition, the thickness can also be a thicknessdetermined for the end of the polishing operation at a particularplaten, i.e., the thickness can be a “final thickness”. Again, the“final thickness” can be an actual thickness or a stand-in for theactual thickness, e.g., the final actual thickness value or final indexvalue, respectively, as discussed above. In some implementations, thepresent index values of the first and second zones are used. In someimplementations, the final index values of the first and second zonesare used.

In some implementations, the first zone is a reference zone and thesecond zone is a control zone. The polishing parameters of the controlzone are modified until the thickness of the second zone falls within apredetermined range of the thickness of the reference zone. In someimplementations there are three or more zones, e.g., five zones, and thefirst zone is the reference zone and the other zones are control zones.An annular middle zone between a circular center control zone and theannular outer control zone can be the reference zone.

It is first determined whether the thickness of the first zone of the atleast two zones falls within a predetermined range of the thickness ofthe second zone of the at least two zones (step 410). In someimplementations, the predetermined range may be determined by polishinga set-up substrate or series of set-up substrates with profiles similarto each other and to the substrate 10 and using polishing conditionssimilar to each other and to the polishing process used to polishsubstrate 10. Data from the set-up substrates may be stored in acomputer. In some implementations, the predetermined range can becalculated from the differences between the two zones, e.g., assumingthe average thickness of the two zones is equal over multiplesubstrates, the range could be based on a standard deviation of thethickness differences over these substrates.

If the thickness of the first zone falls within a predetermined range ofthe thickness of the second zone, e.g., if the final thickness of thefirst zone falls within a predetermined range of the final thickness ofthe second zone, then no modification of the polishing parameters forthe control zone is necessary and the same set of polishing parameterscan be used to polish subsequent additional substrates at the firstplaten. In addition, if the thickness of the first zone falls within apredetermined range of the thickness of the second zone, e.g., if thefinal thickness of the first zone falls within a predetermined range ofthe final thickness of the second zone, then a default set of secondpolishing parameters for the control zone can be used to polish thesubstrate 10 at a second platen.

In some implementations, if the thickness of the first zone does notfall within the predetermined range, e.g., if the final thickness of thefirst zone does not fall within the predetermined range of the finalthickness of the second zone, at least one polishing parameter of thefirst set of polishing parameters for the first zone is adjusted for asubsequent substrate at the first platen to obtain the predeterminedrange during polishing of the subsequent substrate at the first platen(step 412). In some implementations, at least one polishing parameter ofthe control zone is modified for the subsequent substrate at the firstplaten such that the final thickness of the control zone is equivalentto the final thickness of the reference zone at the completion ofpolishing at the first platen. Thus, these implementations can improvewafer-to-wafer polishing uniformity.

In some implementations, if the thickness of the first zone does notfall within the predetermined range, e.g., if the final thickness of thefirst zone does not fall within the predetermined range of the finalthickness of the second zone, at least one polishing parameter of thesecond set of polishing parameters for the first zone is adjusted(relative to the default set of second polishing parameters) for thesubstrate to obtain a second predetermined range between the first zoneand the second zone during polishing of the substrate at the secondplaten (step 412). In some implementations, at least one polishingparameter of the control zone is modified for the substrate at thesecond platen such that the final thickness of the control zone isequivalent to the final thickness of the reference zone at thecompletion of polishing of the substrate at the second platen. Thus,these implementations can improve within-wafer polishing uniformity.

In some implementations, at least one polishing parameter for the firstzone is adjusted in real-time while polishing of the substrate isperformed at the first platen so that the thickness of the control zonebecomes equivalent to the thickness of the reference zone. For example,the polishing parameters of the control zone can be modified such thatsuch that the final thickness of the control zone is equivalent to thefinal thickness of the reference zone at the completion of polishing ofthe substrate at the first platen.

The substrate 10 is then polished on a second platen 24 using either thedefault second set of polishing parameters or adjusted second set ofpolishing parameters (step 414) as determined in step 412. The methodmay be repeated beginning at step 404 where a plurality of measuredspectra is obtained from at least one zone of the substrate 10 (step404). The method may be repeated until the thickness of the first zoneof the substrate falls within the predetermined range.

In some implementations, while the substrate 10 is polished on a secondplaten 24 using the adjusted second set of polishing parameters in afeed forward process (step 414), the adjusted first set of polishingparameters are used to polish a second substrate on the first platen 24in a feed backward process. In some implementations, the informationgained from polishing the second substrate on the first platen 24 may beused to further adjust the second set of polishing parameters whilepolishing the second substrate on the second platen and/or to furtheradjust the first set of polishing parameters while polishing a thirdsubstrate on the first platen. In some implementations, the polishingparameters used to polish the second substrate 10 on the first platen 24are adjusted in real-time based on the polishing of the first substrate10 on the second platen 24 in a combined feed forward and feed backwardprocess.

FIG. 5 illustrates another method 500 for polishing a substrate 10. Thismethod can include the various possible implementations discussed abovefor method 400, but differs as set out below. The method begins bypolishing a first substrate 10 on a first platen 24 using a first set ofpolishing parameters (step 502). A plurality of measured spectra isobtained from at least two zones of the first substrate 10 (step 504).The plurality of measured spectra is compared with a plurality ofreference spectra to evaluate the thickness of each of the at least twozones of the first substrate 10 (step 506). A thickness of a first zoneof the at least two zones of the first substrate is compared with athickness of a second zone of the at least two zones of the firstsubstrate (step 508). It is determined whether the thickness of thefirst zone of the at least two zones of the first substrate falls withina predetermined range of the thickness of the second zone of the atleast two zones of the first substrate, e.g., whether the finalthickness of the first zone falls within a predetermined range of thefinal thickness of the second zone (step 510). If the thickness of thefirst zone does not fall within the predetermined range, at least oneparameter is adjusted to obtain a predetermined range (step 512). Inparticular, at least one parameter from a second set polishingparameters is adjusted from its default setting such that the finalthickness of the control zone is equivalent to the final thickness ofthe reference zone at the completion of polishing of the substrate atthe second platen. In addition, at least one parameter from the firstset polishing parameters is adjusted such that the final thickness ofthe control zone is equivalent to the final thickness of the referencezone at the completion of polishing of a second substrate at the firstplaten. The first substrate 10 is polished on a second platen 24 usingthe adjusted second set of polishing parameters (step 514).

If the thickness of the first zone of the first substrate 10 fallswithin the predetermined range, additional substrates may be polished onthe first platen using the first set of polishing parameters, and thesubstrate may be polished at the second platen using the default secondset of polishing parameters (step 534).

A second substrate 10 is polished on the first platen 24 using theadjusted first set of polishing parameters (step 516). A plurality ofmeasured spectra is obtained from at least two zones of the secondsubstrate 10 (step 518). The plurality of measured spectra is comparedwith a plurality of reference spectra to evaluate the thickness of eachof the at least two zones of the second substrate 10 (step 520). Athickness of a first zone of the at least two zones of the secondsubstrate 10 is compared with a thickness of a second zone of the atleast two zones of the second substrate (step 522). It is determinedwhether the thickness of the first zone of the at least two zones of thesecond substrate falls within a predetermined range of the thickness ofthe second zone of the at least two zones of the second substrate, e.g.,whether the final thickness of the first zone falls within apredetermined range of the final thickness of the second zone (step524).

If the thickness of the first zone of the second substrate 10 fallswithin the predetermined range, additional substrates may be polished onthe first platen using the adjusted first set of polishing parameters(step 536).

If the thickness of the first zone of the second substrate 10 does notfall within the predetermined range, a set of polishing parameters isadjusted to obtain the predetermined range (step 526). In particular, atleast one parameter from the second set of polishing parameters isadjusted from its default setting such that the final thickness of thecontrol zone is equivalent to the final thickness of the reference zoneat the completion of polishing of the second substrate at the secondplaten. In addition, at least one parameter from the first set ofpolishing parameters is re-adjusted such that the final thickness of thecontrol zone is equivalent to the final thickness of the reference zoneat the completion of polishing of a third substrate at the first platen.The second substrate 10 may be polished on the second platen 24 usingthe adjusted second set of polishing parameters (step 528).

If the thickness of the second substrate 10 meets the predeterminedtarget thickness range, it is determined whether there are additionalsubstrates to polish (step 530). If there are additional substrates topolish, a third substrate 10 is polished on the first platen 24 usingthe re-adjusted first set of polishing parameters (step 532).

EXAMPLES

The following non-limiting examples are provided to further illustrateimplementations described herein. These examples can use the techniquesdescribed above. However, the examples are not intended to beall-inclusive and are not intended to limit the scope of theimplementations described herein.

Feed Forward

For Feed Forward techniques, the polishing information obtained frompolishing a first substrate on a platen (x) is used for determining thepolishing parameters to obtain a uniform profile for the same substratesubsequently polished on a second platen (x+1). In some implementations,the Feed Forward techniques may be used to determine the incomingprofile of a substrate. Feed Forward techniques may be used whereadditional substrates do not have an identical incoming profile as thefirst substrate.

FIG. 6A illustrates a graph of the polishing progress versus time for asubstrate polished on a first platen. FIG. 6B illustrates a graph of thepolishing progress versus time for a process in which the polishingparameters for the substrate polished on a second platen are adjustedbased on the information obtained from polishing the substrate on afirst platen. Referring to FIG. 6A, if a particular profile is desired,such as a uniform thickness across the surface of the substrate, theslope of the polishing rate, as indicated by the change in index numbers(y-axis) according to time or platen rotations (x-axis), can bemonitored and the polishing rate adjusted accordingly. Substrate 1 ispolished on platen 1 and spectra are obtained. FIG. 6A illustrates thepolishing information for a reference zone and a control zone onsubstrate 1. Here, the zones are circular and/or annular. Each spectrumis correlated to its respective index. This process is repeated over anumber of platen rotations, or over time, and the polishing rate foreach zone is determined. The polishing rate is indicated by the slope ofthe line that is obtained by plotting the index (y-axis) versus time(x-axis).

Referring to FIG. 6A, R_(E1) represents the endpoint index value for thereference zone and T_(E1) represents the time at which the endpoint forthe reference zone is reached. Line 610 represents the robust fit linefor the reference zone. R_(ES1) represents the starting index value forthe reference zone, T_(ZS1) represents the actual polishing start timeof the control zone of substrate 1 on platen 1, and T_(Z1) representsthe time that the control zone reaches the endpoint index value(R_(E1)). Line 620 represents the robust line fit for the control zoneon the substrate. T_(ZS1) represents the effective time that the controlzone achieves the starting index value R_(ES1) of the reference zone.Although the control zone stops polishing when line 620 reaches theendpoint time T_(E1) for the reference zone, line 620 may beextrapolated to show where line 620 intersects with the endpoint indexvalue R_(E1), which is represented on the x-axis by T_(Z1). Thedifference between T_(Z1) and T_(E1) represents the additional polishingtime for the control zone to achieve the same thickness as the referencezone.

Referring to FIG. 6B, R_(E2) represents the endpoint index value for thereference zone on platen 2 and T_(E2) represents the time at which theendpoint for the reference zone is reached on platen 2. Line 630represents a previously determined polishing progress for the referencezone, e.g., the robust line fit for the reference zone from polishing ofa prior substrate, e.g., the test substrate using default polishingparameters. M_(E2) is the slope of the line 630. R_(ES2) represents thestarting index value for the reference zone on platen 2, T_(ZS2)represents the effective polishing start time for the control zone ofsubstrate 1 on platen 2, i.e., the time at which the starting indexvalue R_(ES2) for the reference zone should be achieved by the controlzone. Line 640 represents the desired polishing progress for the controlzone and the reference zone to converge on R_(E2) at the same time.M_(ZD2) is the slope of the line 640.

The polishing process on platen 1 can be different than the polishingprocess on platen 2, for example, the polishing process on platen 1polishes at a faster rate than the polishing process on platen 2. Forexample, it can take 20 rotations for platen 1 to remove 1000 Å ofmaterial, while it can take platen 2 40 rotations to remove 1000 Å ofmaterial. As a result of the differing polishing processes, thedifference in thickness between the reference zone and the control zonefrom platen 1 is related to the difference in rotation rates betweenplaten 1 and platen 2. T_(ZS2) is calculated as follows:((RR₂/RR₁)(T_(Z1)−T_(E1))) where RR₁ represents the rotation rate ofplaten 1, RR₂ represents the rotation rate of platen 2, and T_(Z1) andT_(E1) were both determined for platen 1. M_(ZD2), the slope of line 640which represents the desired polishing rate for the control zone toconverge on R_(E2), may be calculated as follows:((R_(E2)/(T_(EZ)−T_(ZS2)))). P_(ZNEW), represents the polishing pressurethat should be used on platen 2 to achieve a uniform polishing profilebetween the control zone and the reference zone, may be calculated asfollows: ((M_(ZD2)/M_(E2))(P_(ZOLD))). In some implementations, P_(ZOLD)represents polishing pressure used for polishing the reference zone onplaten 2. In some implementations, P_(ZOLD) represents the polishingpressure used for polishing the control zone on platen 1. In someimplementations, P_(ZOLD) represents a default the polishing pressureused for polishing the control zone on platen 2.

Feed Back

For Feed Back techniques, the polishing information obtained frompolishing a first substrate on a platen is used for determining thepolishing parameters to obtain a uniform profile for a second substratesubsequently polished on the platen. Feed Back techniques assume thatthe incoming substrate has the same incoming profile as the previoussubstrate.

FIG. 7A illustrates a graph of the polishing progress versus time for afirst substrate polished on a platen. FIG. 7B illustrates a graph of thepolishing progress versus time for a process in which the polishingrates are adjusted for a second substrate polished on the platen.Referring to FIG. 7A, if a particular profile is desired, such as auniform thickness across the surface of the substrate, the slope of thepolishing rate, as indicated by the change in index numbers (y-axis)according to time or platen rotations (x-axis), can be monitored and thepolishing rate adjusted accordingly. Substrate 1 is polished on platen 1and spectra are obtained. Although the spectra may be obtained formultiple zones on the substrate, FIG. 7A shows the polishing informationfor a reference zone and a control zone. Here, the zones are annular orcircular. Each spectrum is correlated to its respective index. Thisprocess may be repeated over a number of platen rotations, or over time,and the polishing rate for each zone is determined.

The polishing rate is indicated by the slope of the line that isobtained by plotting the index (y-axis) versus time (x-axis). Line 710represents the robust fit line for the reference zone. The slope M_(E1),determined from the slope of line 710, is the polishing rate of thereference zone on the substrate obtained during the polishing ofsubstrate 1. R_(ES1) represents the starting index value for thereference zone, R_(E1) represents the target index value for thereference zone to halt polishing, T_(ES1) represents the actualpolishing start time of the reference zone, and T_(E1) represents thetime that the reference zone reaches the endpoint index value (R_(E1)).Line 720 represents the robust line fit for the control zone on thesubstrate. The slope M_(ZA1), determined from the slope of line 720, isthe actual polishing rate of the control zone on the substrate obtainedduring the polishing of substrate 1. R_(ZS1) represents the startingindex value for the control zone, R_(Z1) represents the index value ofthe control zone when polishing is halted, T_(ZS1) represents thepolishing start time for the control zone, and T_(Z1) represents aprojected time that the control zone would reach the target index value(R_(E1)). The slope M_(ZD1), represented by line 730, is the desiredpolishing rate of the control zone such that the control zone convergeson the endpoint index value (R_(E1)) of the reference zone at the timeT_(E1). Slope M_(ZD1) represents the desired polishing rate in order toachieve the desired endpoint represented by the intersection of R_(E1)and T_(E1). Slope M_(ZD1) can be calculated using the informationobtained from M_(E1) and M_(ZA1). Specifically, the slope M_(ZD1) can becalculated as ((R_(E1)−R_(ZS1))/(T_(E1)−T_(ZS1)).

Assuming an identical incoming profile for substrate 2 on platen 1, thepolishing rate information from substrate 1 on platen 1 is fed backwardand used to determine a new polishing pressure (P_(ZNEW)). P_(ZNEW)represents the polishing pressure that the control zone of the secondsubstrate should be polished at in order for the control zone toconverge on the reference zone. P_(ZNEW) is calculated as follows:((M_(ZD1)/M_(ZA1))×P_(ZOLD)) where P_(ZOLD) represents the polishingpressure for the control zone on substrate 1. Polishing substrate 2 atP_(ZNEW) provides a more uniform polish allowing the control zone toconverge on the reference zone of substrate 2. Gain factors and othercontrol techniques can be applied to dampen or amplify the newrecommended pressure (P_(ZNEW)).

Feed Forward/Feed Back

Feed back techniques help compensate for changes in removal rates but donot account for differences in incoming thickness information. Feedforward techniques compensate for differences in incoming thickness onplaten 2 but do not account for differences in removal rates. Therefore,in certain situations it is desirable to combine feed forward techniqueswith feed backward techniques to achieve a uniform polishing profile.

In some implementations, the new pressure P_(NEW) for the control zone,e.g., at platen 2, is calculated as: P_(NEW)=P_(INITIAL)+P_(FB)+P_(FF),where P_(INITIAL) is a preset pressure for the control zone, e.g., aconstant, e.g., the best-known-method (BKM) pressure, P_(FB) is thefeedback contribution to the pressure, and P_(FF) is the feed-forwardcontribution to the pressure. P_(FB) accounts for variations in thepolishing rates of the zones due to process impacts, e.g., pad life, atthe platen at which the substrate is being polished. P_(FF) accounts fordifferent incoming thickness variations between zones, e.g., asdetermined by optical monitoring at platen 1.

As discussed above, for each of the reference and control zones, linescan be fit to the sequence of values for the respective zones. Inaddition, as discussed above, the values can be index values and thetime can be measured in platen rotations.

The feed back contribution P_(FB) can be calculated as follows:P_(FB)=k1*(P_(INITIAL))*((T_(C2)−T_(R2))/T_(C2)), where k1 is apre-defined constant, e.g., 1, T_(R2) is the time that the fitted linefor the reference zone crossed the target value for the previoussubstrate at platen 2, and T_(C2) is the time or projected time that thefitted line for the control zone crossed the target value for theprevious substrate at platen 2 (both T_(C2) and T_(R2) are relative tothe start of polishing for the previous substrate at platen 2).

The feedback value can be weighted using common moving average orexponential weight moving average. An example of moving average (MA)calculation for the feedback contribution P_(FB)(n+1) to the nextsubstrate is the following:MA[P_(FB)(n+1)]=λP_(FB)(n)+(1−λ)MA[P_(FB)(n−1)], where is the newestfeedback contribution, P_(FB)(n) is a feedback contribution ascalculated above for the current substrate, P_(FB)(n−1), is the movingaverage feedback contribution for the previous substrate, and λ is aconstant.

The feed forward contribution P_(FF) can be calculated as follows:P_(FF)=k2*(P_(INITIAL))*((R_(C1)−R_(R1))/RR₂)T_(C2), where k2 is apre-defined constant, e.g., 1, R_(R1) is the value of the fitted linefor the reference zone for the substrate at the endpoint time at platen1, e.g., the target value at platen 1, R_(C1) is the value or projectedvalue for the fitted line for the control zone at the endpoint time forthe substrate at platen 1, and RR₂ is the rotation rate of platen 2(both R_(C1) and R_(R1) can be relative to the value of the referencezone at the start of polishing at platen 1).

The methods and functional operations described in this specificationcan be implemented in digital electronic circuitry, or in computersoftware, firmware, or hardware, including the structural meansdisclosed in this specification and structural equivalents thereof, orin combinations of them. The methods and functional operations can beperformed by one or more computer program products, i.e., one or morecomputer programs tangibly embodied in an information carrier, e.g., ina computer readable media, such as a machine readable storage device, orin a propagated signal, for execution by, or to control the operationof, data processing apparatus, e.g., a programmable processor, acomputer, or multiple processors or computers. A computer program (alsoknown as a program, software, software application, or code) can bewritten in any form of programming language, including compiled orinterpreted languages, and it can be deployed in any form, including asa stand alone program or as a module, component, subroutine, or otherunit suitable for use in a computing environment. A computer programdoes not necessarily correspond to a file. A program can be stored in aportion of a file that holds other programs or data, in a single filededicated to the program in question, or in multiple coordinated files(e.g., files that store one or more modules, sub programs, or portionsof code). A computer program can be deployed to be executed on onecomputer or on multiple computers at one site or distributed acrossmultiple sites and interconnected by a communication network.

The processes and logic flows described in this specification can beperformed by one or more programmable processors executing one or morecomputer programs to perform functions by operating on input data andgenerating output. The processes and logic flows can also be performedby, and apparatus can also be implemented as, special purpose logiccircuitry, e.g., an FPGA (field programmable gate array) or an ASIC(application specific integrated circuit).

The substrate can be, for example, a product substrate (e.g., whichincludes multiple memory or processor dies), a test substrate, and agating substrate. The substrate can be at various stages of integratedcircuit fabrication, e.g., the substrate can include one or moredeposited and/or patterned layers. The term substrate can includecircular disks and rectangular sheets. The deposited and/or patternedlayers can include insulative materials, conductive materials, andcombinations thereof. In implementations where the material is aninsulative material, the insulative material can be an oxide, e.g.,silicon oxide, a nitride, or another insulative material used in theindustry to produce electronic devices. In implementations where thematerial is a conductive material, the conductive material can be acopper containing material, tungsten containing material, or anotherconductive material used in the industry to produce electronic devices.

While the foregoing is directed to various implementations, other andfurther implementations may be devised, and the scope of the inventionis determined by the claims that follow.

1. A method of controlling the chemical mechanical polishing of asubstrate, comprising: polishing a substrate on a first platen using afirst set of polishing parameters; obtaining a first sequence ofmeasured spectra with an in-situ optical monitoring system, eachmeasured spectrum from the first sequence of measured spectra being aspectrum of light reflected from a first region of the substrate;generating a first sequence of values from the first sequence ofmeasured spectra; fitting a first linear function to the first sequenceof values; obtaining a second sequence of measured spectra with thein-situ optical monitoring system, each measured spectrum from thesecond sequence of measured spectra being a spectrum of light reflectedfrom a second region of the substrate; generating a second sequence ofvalues from the second sequence of measured spectra; fitting a secondlinear function to the second sequence of values; determining adifference between the first linear function and the second linearfunction; adjusting at least one polishing parameter of a second set ofpolishing parameters based on the difference so as to reduce an expecteddifference between an expected first linear function for the firstregion and an expected second linear function for the second regionduring polishing of the substrate on a second platen; and polishing thesubstrate on the second platen using the adjusted polishing parameter.2. The method of claim 1, wherein the first region comprises a controlregion and the second region comprises a reference region, and polishingof the first substrate at the first platen is halted based on the secondlinear function reaching a target value.
 3. The method of claim 2,wherein determining the difference comprises determining a difference intime between a first time that the first linear function reaches thetarget value and a second time that the second function reaches thetarget value.
 4. The method of claim 3, wherein adjusting the at leastone polishing parameter includes multiplying the difference in time by aratio of a rotation rate of the first platen to a rotation rate of thesecond platen.
 5. The method of claim 3, wherein the polishing parameteris pressure.
 6. The method of claim 5, wherein adjusting the at leastone polishing parameter includes calculating an expected polishing ratefor the substrate at the second platen from a second target value, asecond target time and the difference in time.
 7. The method of claim 6,wherein MZD2=RE2/(TE2−TZS2), where MZD2 is the expected polishing rate,RE2 is the second target value, TE2 is the second target time, and TZS2is the difference in time multiplied by the ratio of the rotation rateof the first platen to the rotation rate of the second platen.
 8. Themethod of claim 7, wherein adjusting the polishing rate comprisesmultiplying the pressure by a ratio of the expected polishing rate to aprior polishing rate.
 9. The method of claim 8, wherein the pressure isa pressure for the reference region.
 10. The method of claim 2, whereinadjusting the at least one polishing parameter comprises adjusting apolishing parameter for the control region.
 11. The method of claim 3,wherein the difference in time is measured as a difference in platenrotations.
 12. The method of claim 1, wherein generating a firstsequence of values comprises, for each measured spectrum, determining abest fitting first reference spectrum from a library of referencespectra, and determining a first value associated with the best fittingreference spectrum.
 13. The method of claim 12, wherein the first valuecomprises an index value.
 14. The method of claim 12, wherein the firstvalue comprises a thickness value.
 15. The method of claim 12, whereingenerating the second sequence of values comprises, for each measuredspectrum from the second sequence, determining a best fitting secondreference spectrum from the library of reference spectra, anddetermining a second value associated with the best fitting secondreference spectrum.
 16. The method of claim 1, wherein obtaining thefirst sequence of measured spectra and the second sequence of measuredspectra includes illuminating the substrate with white light andobtaining spectra of light reflected from the substrate.
 17. A computerprogram product, tangibly embodied on a computer readable storagedevice, comprising instructions for causing a processor to control achemical mechanical polisher to perform operations of: polishing asubstrate on a first platen using a first set of polishing parameters;obtaining a first sequence of measured spectra with an in-situ opticalmonitoring system, each measured spectrum from the first sequence ofmeasured spectra being a spectrum of light reflected from a first regionof the substrate; generating a first sequence of values from the firstsequence of measured spectra; fitting a first linear function to thefirst sequence of values; obtaining a second sequence of measuredspectra with the in-situ optical monitoring system, each measuredspectrum from the second sequence of measured spectra being a spectrumof light reflected from a second region of the substrate; generating asecond sequence of values from the second sequence of measured spectra;fitting a second linear function to the second sequence of values;determining a difference between the first linear function and thesecond linear function; adjusting at least one polishing parameter of asecond set of polishing parameters based on the difference so as toreduce an expected difference between an expected first linear functionfor the first region and an expected second linear function for thesecond region during polishing of the substrate on a second platen; andpolishing the substrate on the second platen using the adjustedpolishing parameter.
 18. The computer program product of claim 17,wherein the first region comprises a control region and the secondregion comprises a reference region, and polishing of the firstsubstrate at the first platen is halted based on the second linearfunction reaching a target value, wherein adjusting the at least onepolishing parameter includes determining a difference between a firsttime that the first linear function reaches the target value and asecond time that the second function reaches the target value,calculating an expected polishing rate for the substrate at the secondplaten, wherein MZD2=RE2/(TE2−TZS2), where MZD2 is the expectedpolishing rate, RE2 is a second target value, TE2 is a second time, andTZS2 is the difference in time multiplied by the ratio of the rotationrate of the first platen to the rotation rate of the second platen, andmultiplying a pressure by a ratio of expected polishing rate to a priorpolishing rate.
 19. The computer program product of claim 17, whereingenerating the first sequence of values comprises, for each measuredspectrum from the first sequence, determining a best fitting firstreference spectrum from a library of reference spectra, and determininga first value associated with the best fitting first reference spectrum,and wherein generating the second sequence of values comprises, for eachmeasured spectrum from the second sequence, determining a best fittingsecond reference spectrum from the library of reference spectra, anddetermining a second value associated with the best fitting secondreference spectrum.
 20. The method of claim 17, wherein the values ofthe first sequence and second sequence comprise index values.
 21. Asystem for chemical mechanical polishing, comprising: a rotatable firstplaten for supporting a polishing surface; a light source in the firstplaten; a light detector in the first platen; a rotatable second platenfor supporting a second polishing surface; a carrier head configured tohold a substrate against the polishing surface and move the substrate sothat light from the light source is directed onto the surface of thesubstrate and light reflected from the substrate is detected by thelight detector; and a controller configured to receive a signal from thelight detector, wherein the controller is further configured to polishthe substrate at the first platen, obtain a first sequence of measuredspectra with an in-situ optical monitoring system, each measuredspectrum from the first sequence of measured spectra being a spectrum oflight reflected from a first region of the substrate, generate a firstsequence of values from the first sequence of measured spectra, fit afirst linear function to the first sequence of values, obtain a secondsequence of measured spectra with the in-situ optical monitoring system,each measured spectrum from the second sequence of measured spectrabeing a spectrum of light reflected from a second region of thesubstrate, generate a second sequence of values from the second sequenceof measured spectra, fit a second linear function to the second sequenceof values; determine a difference between the first linear function andthe second linear function, adjust at least one polishing parameter of asecond set of polishing parameters based on the difference so as toreduce an expected difference between an expected first linear functionfor the first region and an expected second linear function for thesecond region during polishing of the substrate on the second platen,and polish the substrate on the second platen using the adjustedpolishing parameter.
 22. The system of claim 21, wherein the firstregion comprises a control region and the second region comprises areference region, and polishing of the first substrate at the firstplaten is halted based on the second linear function reaching a targetvalue, wherein adjusting the at least one polishing parameter includesdetermining a difference between a first time that the first linearfunction reaches the target value and a second time that the secondfunction reaches the target value, calculating an expected polishingrate for the substrate at the second platen, whereinMZD2=RE2/(TE2−TZS2), where MZD2 is the expected polishing rate, RE2 is asecond target value, TE2 is a second time, and TZS2 is the difference intime multiplied by a ratio of the rotation rate of the first platen tothe rotation rate of the second platen, and multiplying a pressure by aratio of the expected polishing rate to a prior polishing rate.
 23. Thesystem of claim 21, wherein generating the first sequence of valuescomprises, for each measured spectrum from the first sequence,determining a best fitting first reference spectrum from a library ofreference spectra, and determining a first value associated with thebest fitting first reference spectrum, and wherein generating the secondsequence of values comprises, for each measured spectrum from the secondsequence, determining a best fitting second reference spectrum from thelibrary of reference spectra, and determining a second value associatedwith the best fitting second reference spectrum.
 24. The method of claim21, wherein the values of the first sequence and second sequencecomprise index values.